The present invention relates generally to control valves of the type commonly utilized in vehicular power steering systems and, more particularly, to such control valves having hydraulically balanced flow passages wherein input flow pressure loss through the input flow control orifices is substantially identical to return flow pressure loss through the return flow control orifices.
In power steering systems, it is desirable for the control valve to incorporate a "hydraulically balanced" design wherein the pressure loss associated with input flow is substantially equal to the pressure loss associated with return flow for virtually all valve displacement values. According to such a balanced arrangement, the control valve is capable of providing superior performance characteristics while concurrently minimizing parasitic power losses. However, most control valves used in conventional vehicular power steering systems are not hydraulically balanced, but rather are constructed in a "geometrically" balanced manner since the direction of fluid flow through the input and return flow control orifices are opposite with respect to one another. By way of example, the power steering valves described in SAE Paper 880707 and entitled 1988 "LINCOLN CONTINENTAL VARIABLE-ASSIST POWER STEERING SYSTEM" and U.S. Pat. No. 4,651,551 entitled "MEANS FOR MANUFACTURING COMPONENTS OF ROTARY VALVES", issued to Bishop et al. on Mar. 24, 1987 have input and return flow control orifices that are formed by the tangential spaces that open between the chamfered edges of the valve spool slots and the nominally orthogonal edges of the valve sleeve slots. As is known, the edges of the valve spool slots are chamfered to selectively modify flow characteristic and, more particularly, to minimize audible "hissing" noise. Due to the oppositely directed fluid flow through the input and return flow control orifices, the boundaries (i.e., "streamlines") of the fluid jets issuing therefrom differ significantly.
In most conventional control valves, the fluid entering the control valve flows through the input flow control orifices from inlet slots and is channeled by the confluence of the edge chamfers and a circumferential surface to first and second sets of output slots. Concurrently, the flow returning from the first and second sets of output slots passes through the return flow control orifices and into the return slots. When utilized, the chamfered edges of the valve spool slots are geometrically identical to each other. Thus, the fluid jets issuing from the slots formed in the valve spool issue freely into the slots formed in the valve sleeve. However, the fluid jets issuing from the valve sleeve slots do not issue freely into the valve spool slots. Rather, the resulting fluid jet impinges upon the chamfered edges of the valve spool slots at a position and intensity determined as a function of the degree of tangential valve deflection between the valve spool and valve sleeve. This action results in a flow condition wherein partial "flow back" into the flow control orifice occurs which detrimentally reduces its overall working area.
Accordingly, the present invention is directed to providing an improved control valve having flow passages that are substantially "hydraulically balanced" by virtue of each having a similar flow configuration for fluid flow through the input and return flow control orifices. According to a first preferred construction of the control valve, the flow passages are formed with slot walls that serve to configure the flow control orifices with nominally uniform converging angles. This construction serves to substantially inhibit the jet flow issuing from either input or return flow control orifice from impinging upon a surface in juxtaposition to the flow control orifice itself. Thus, the possibility of partial "flow back" into the flow control orifice is eliminated which, in turn, also prevents any undesirable reduction of the flow control orifice area.
In an another preferred embodiment of the present invention, a novel method of forming valve sleeve slots is presented. According to this method, the valve sleeve slots are formed as portions of elongated cylindrical bores formed in a valve sleeve blank in a manner similar to the chambers of a revolver. After the cylindrical bores are formed, an intermediate bore and a counterbore are formed within the valve sleeve blank. The intermediate bore laterally exposes the bores leaving only the valve sleeve slots while the counterbore serves to terminate the axial extent of the slots. A sealing ring is inserted in the counterbore and an enlarged uniform final bore is formed through both the valve sleeve blank and sealing ring to complete the inside configuration of the valve sleeve assembly.
Other features, objects and advantages of the present invention will become readily apparent to one skilled in the art upon analysis of the following written description taken in conjunction with the accompanying drawings and appended claims.